The present invention relates generally to processes for producing organic acids, such as lactic acid.
Lactic acid has a number of commercial uses, for example in food manufacturing, pharmaceuticals, plastics, textiles, and as a starting material in various chemical processes. In addition, it is used in the manufacture of polylactic acid, a degradable plastic.
Although organic acids can be prepared by chemical synthesis, production by fermentation is generally less expensive. It is well known to produce lactic acid by fermentation using microorganisms such as Lactobacillus delbrueckii. The broth that results from fermentation contains unfermented sugars, carbohydrates, amino acids, proteins, and salts, as well as organic acids, such as lactic acid. Typically, the organic acid is recovered from the fermentation broth and undergoes further purification before it is used. Purified organic acids recovered from fermentation broths can comprise small amounts of impurities, such as strong acids or certain unknown compounds. Some of these impurities can cause an undesirable color or can interfere with downstream processing of the organic acid. For example, lactic acid as it is sold commercially typically comprises small amounts of impurities such as pyruvic acid and oxalic acid. Even though present in relatively small amounts, such impurities can have negative effects on polymers produced from the lactic acid. For example, when lactic acid is polymerized to produce polylactic acid (PLA), the presence of even small amounts of pyruvic acid can cause the polymer to have an undesirable yellow color. However, it is difficult to further purify lactic acid that contains only a small fraction of pyruvic acid in the first instance.
Thus, there is a need for improved processes for the production and recovery of relatively pure organic acids, particularly lactic acid.
One aspect of the present invention is a process for purifying an aqueous feed stream that comprises a desired product organic acid and at least one strong contaminant. In certain embodiments, the aqueous feed stream can comprise a fermentation broth or can be obtained from a fermentation broth. (Whenever an acid is referenced herein, either as the desired product or as a contaminant, it should be understood that some or all of the acid may be present in the form of salts.) The molar concentration of the product organic acid in the feed stream can be at least 10 times greater than the molar concentration of the strong contaminant, and more preferably the molar concentration of the product organic acid to the strong contaminant is at least 20. In certain embodiments the molar concentration of the product organic acid to the strong contaminant is at least 90, in certain embodiments it is at least 500 and in certain embodiments it is at least 1000. The aqueous feed stream is contacted with a first immiscible basic extractant that has a selectivity, under the existing process conditions (including the combination of acids, solvents, etc., that are present) for the strong contaminant relative to the product organic acid that is greater than 3. The selectivity, which is further defined below, is preferably greater than 15, more preferably greater than about 25, most preferably greater than about 100. Preferably the selectivity is greater than the ratio of product organic acid to strong contaminant in the feed.
The contacting step in which the aqueous feed stream is contacted with a first immiscible basic extractant is preferably performed with sufficient equilibrium or near equilibrium stages, and with sufficient quantity of the first immiscible basic extractant (such as a solid amine ion exchanger or liquid amine extractant) to remove the majority of the strong contaminant. In certain embodiments the first immiscible basic extractant has previously been used to treat a solution comprising the product organic acid and at least one weak contaminant (e.g., it is recycled).
As a result, the majority of the strong contaminant and less than about 33 wt % of the product organic acid become complexed with the first immiscible basic extractant. xe2x80x9cMajorityxe2x80x9d as used herein means more than 50% by weight of the substance, in this case the strong contaminant, that is present. In other words, more than 50% by weight of the strong contaminant present in the feed complexes with the extractant. The complexed first immiscible basic extractant is separated from the aqueous stream, thereby producing a first effluent stream that comprises product organic acid and that has a greater ratio of product organic acid to strong contaminant than the aqueous feed stream did. The complexed first immiscible basic extractant is contacted with a displacing acid. The first immiscible basic extractant has a greater affinity for the displacing acid than it does for the strong contaminant or the product organic acid, and as a result, product organic acid and strong contaminant are displaced over a period of time from the complexed first immiscible basic extractant. This produces a second effluent stream that comprises a major amount of product organic acid (i.e., more than 50% by weight of the solids dissolved or suspended in the stream are the product organic acid) and a third effluent stream that comprises a major amount of strong contaminant. Preferably, the total amount of product organic acid present in the first effluent stream and in the second effluent stream is at least about 90% by weight of the product organic acid that was present in the feed stream. More preferably, at least about 98% by weight of the product organic acid is recovered in those streams.
In many embodiments of the process, the strong contaminant comprises an organic acid that has a pKa that is lower than the pKa of the product organic acid. If the desired product organic acid is lactic acid, the strong contaminant preferably has a pKa less than about 3.46. In certain specific embodiments of the process involving a basic extractant that is a solid ion exchange resin, the strong contaminant is selected from the group consisting of pyruvic acid, oxalic acid, citraconic acid, citric acid, and mixtures thereof.
In other embodiments, the strong contaminant can be a weaker acid (e.g., higher pKa) than the desired organic acid product, but can have greater hydrophobic and/or hydrogen bonding character than the product. The strong contaminant is selectively removed relative to the organic acid of interest by an immiscible basic extractant comprising a solvent or a solvent mixture, for example an amine mixture comprising 1 M trilaurylamine and 1 M dodecanol with dodecane as a diluent.
When the immiscible basic extractant comprises a solvent mixture, preferably the organic acid of interest has somewhat hydrophobic or strong hydrogen bonding characteristics, and the strong contaminant must either (1) be of similar H-bonding and/or hydrophobic character and lower pKa than the acid of interest (acidic low pKa species) or (2) have a sufficiently stronger H-bonding and/or hydrophobic character so that the strong contaminant can still be removed despite its having a higher relative pKa. Thus, if the strong contaminant has a lower pKa than the organic acid product, a solid ion exchange resin can be used as an extractant to remove the strong contaminant. For example, if the organic acid to be recovered is lactic acid, strong contaminants that can be removed by methods of the present invention involving ion exchange resins include HCl, H2SO4, pyruvic acid and oxalic acid, among others. Acetic acid and butyric acid do not, however, have significantly lower pKa values than lactic acid, and they are not as readily removed by ion exchange resins. However strong contaminants having either low pKa or high hydrophobicity/hydrogen bonding characteristics relative to the organic acid product, can be removed using extractants that are solvents or a solvent mixture. For example, pyruvic acid, H2SO4, and butyric acid can be removed from an aqueous feed stream comprising lactic acid with use of an amine solvent extractant of the present invention. Although the first immiscible basic extractant can take various forms, one that is preferred is a weak base ion exchange resin. Preferably the weak base ion exchange resin comprises a tertiary amine moiety. One of the advantages of many embodiments of this process is the ability to further purify a stream that already contains a very low percentage of impurities. For example, in one embodiment, the molar concentration of lactic acid, e.g., the product organic acid, in the feed stream is at least 20 times greater than the molar concentration of the strong contaminant in the feed stream, and the selectivity is greater than about 25. In another embodiment, the molar concentration of the lactic acid, e.g., the product organic acid, in the feed stream is at least 300 times greater than the molar concentration of the strong contaminant in the feed stream, and the selectivity is greater than about 500.
In one embodiment of the process, the feed stream, the first effluent stream, and the second effluent stream further comprise a weak contaminant. For example, where the product organic acid is lactic acid, the weak contaminant can be an organic acid having a pKa greater than about 4.26, such as propionic acid, butyric acid, malonic acid, succinic acid, or mixtures thereof. In this situation, the process can further comprise the steps of combining the first effluent stream and the second effluent stream to form a combined product organic acid stream, and then contacting the combined product organic acid stream with a second immiscible basic extractant. The majority of the product organic acid becomes complexed with the second immiscible basic extractant. Preferably, at least 90% by weight of the product acid becomes complexed, more preferably at least 95%. The complexed second immiscible basic extractant can then be separated from the stream, thereby producing a fourth effluent stream that comprises the majority of the weak contaminant that was present in the combined product organic acid stream. Preferably the second immiscible basic extractant comprises a weak or strong base ion exchange resin.
Optionally, this embodiment of the process can further comprise contacting the fourth effluent stream with a third immiscible basic extractant that has a greater affinity for the product organic acid than for the weak contaminant, whereby the majority of the product organic acid that is present in the fourth effluent stream becomes complexed with the third immiscible basic extractant. The complexed third immiscible basic extractant can then be separated from the stream, thereby producing a fifth effluent that comprises the majority of the weak contaminant that was present in the combined product organic acid stream. Then the complexed second immiscible basic extractant and the complexed third immiscible basic extractant can be contacted with one or more displacing acids, thereby displacing product organic acid therefrom in one or more additional effluent streams.
In another variation of the process, the third effluent stream is contacted with an additional immiscible basic extractant that has a greater affinity for the strong contaminant than for the product organic acid. As a result, the majority of the strong contaminant present in the third effluent stream becomes complexed with the additional immiscible basic extractant. The complexed additional immiscible basic extractant is separated from the remaining stream, thereby producing an additional effluent that comprises the majority of the product organic acid that was present in the third effluent.
One particularly preferred embodiment of the invention is a process for purifying lactic acid. The embodiment involves providing an aqueous feed stream comprising lactic acid (defined herein to include any salts thereof) and at least one strong contaminant acid having a pKa less than about 3.46. The molar concentration of lactic acid in the feed stream is at least 20 times greater than the molar concentration of the strong contaminant acid. The aqueous feed stream is contacted with a first basic ion exchanger that has a greater affinity for the strong contaminant acid than for lactic acid, such that the majority of the strong contaminant acid and some lactic acid become complexed with the first basic ion exchanger. The complexed first basic ion exchanger is separated from the aqueous stream, producing a first effluent stream that comprises lactic acid and that has a greater ratio of lactic acid to strong contaminant acid than the aqueous feed stream did. The complexed first basic ion exchanger is contacted with a displacing acid, such as HCl, H2SO4, or H3PO4, and the first basic ion exchanger has a greater affinity for the displacing acid than it does for the strong contaminant acid or lactic acid. Lactic acid and strong contaminant acid are displaced over a period of time from the complexed first basic ion exchanger, producing a second effluent stream that comprises a major amount of lactic acid, and a third effluent stream that comprises a major amount of strong contaminant acid.
In some embodiments of this process, the strong contaminant acid is selected from the group consisting of pyruvic acid, oxalic acid, citraconic acid, citric acid, and mixtures thereof. In certain embodiments of the process, the molar concentration of lactic acid in the feed stream is at least 100 times greater than the molar concentration of the strong contaminant acid in the feed stream and the selectivity is greater than about 250. In certain embodiments, the ratio is at least 300 and the selectivity is greater than about 500.
Preferably, the first basic ion exchanger has an affinity for the displacing acid that is at least 10 times greater than its affinity for pyruvic acid.
Preferably, the first effluent stream and the second effluent stream collectively have a ratio of lactic acid to strong contaminant that is greater than 300. More preferably, they collectively have a ratio of molar lactic acid to molar strong contaminant that is greater than about 1,000. In some embodiments of the process, at least 90%, and more preferably 98% by weight of the lactic acid present in the feed stream is recovered in the first effluent stream and the second effluent stream.
In a particular embodiment of the process, the aqueous feed stream comprises no more than about 0.15 moles of cations selected from the group consisting of Ca, Mg, Na, Fe, Zn, Zr, and Li, per mole of lactic acid; no more than about 0.05 moles of anions selected from the group consisting of Cl, SO4, PO4, and NO3, per mole of lactic acid; nor more than about 0.03 mole of strong acid contaminants selected from the group consisting of pyruvic acid, oxalic acid, citraconic acid, and citric acid, per mole of lactic acid; and no more than about 0.02 mole of weak acid contaminants selected from the group consisting of propionic acid, butyric acid, malonic acid, and succinic acid, per mole of lactic acid.
Another aspect of the invention is a process for purifying lactic acid involving providing an aqueous fermentation broth comprising lactic acid (defined herein to include any salts thereof) and pyruvic acid (also defined herein to include any salts thereof). The molar concentration of the lactic acid is at least 20 times greater than the molar concentration of pyruvic acid in the aqueous fermentation broth. Cells are removed from the broth to form an aqueous feed stream. Any method known in the art for removing cells can be used (e.g., centrifugation or filtration, among others). The aqueous feed stream is contacted with means for complexing pyruvic acid, and the means has a greater affinity for pyruvic acid than for lactic acid, so that the majority of the pyruvic acid and some lactic acid form complexes therewith. The complexes are separated from the aqueous stream, thereby producing a first effluent stream that comprises lactic acid and that has a greater ratio of lactic acid to pyruvic acid than the aqueous feed stream did. The complexes are contacted with means for displacing lactic acid and pyruvic acid therefrom, thereby producing a second effluent stream that comprises a major amount of lactic acid and a third effluent stream that comprises a major amount of pyruvic acid.
In certain embodiments the first immiscible basic extractant is a solid basic extractant, and the aqueous feed stream is contacted with the first immiscible basic extractant in a packed bed. Preferably in embodiments involving a packed bed, the ratio of molar concentration of product organic acid to molar concentration of strong contaminant in the second effluent stream is within about 10% of the selectivity.
In other embodiments, the first immiscible basic extractant is a liquid basic extractant, and the aqueous feed stream is contacted with the first immiscible basic extractant in a multistage process. Preferably the ratio of product organic acid to strong contaminant is within about 10% of the selectivity.
Furthermore, in certain embodiments the aqueous feed stream comprises more than one strong contaminant, and at least one of the strong contaminants is a displacing acid.
Certain embodiments are directed to a process involving an aqueous feed stream that comprises a desired product organic acid, at least one strong contaminant, and a weak contaminant. The first effluent stream also comprises the weak contaminant, and the first effluent stream is processed by at least one of extraction or distillation (by methods known in the art), whereby at least two fractions are produced, a purified product organic acid fraction comprising between about 90% and 99.5% by weight of the product organic acid that was present in the feed stream, and a weak contaminant fraction comprising product organic acid and weak contaminant. The ratio of molar concentration of product organic acid to molar concentration of weak contaminant in the weak contaminant fraction is less than the ratio of molar concentration of product organic acid to molar concentration of weak contaminant in the feed stream. The weak contaminant fraction is contacted with a third immiscible basic extractant that has a selectivity for the product organic acid relative to the weak contaminant that is greater than about 3, and the majority of the product organic acid and less than about 33 wt % of the weak contaminant become complexed with the third immiscible basic extractant. The complexed third immiscible basic extractant is separated from the aqueous stream, to produce an effluent stream that comprises weak contaminant. The effluent stream that comprises weak contaminant has a greater ratio of weak contaminant to product organic acid than the aqueous feed stream did. The complexed third immiscible basic extractant is contacted with a displacing acid, and the third immiscible basic extractant has a greater affinity for the displacing acid than it does for the product organic acid or the weak contaminant. The displacing acid is present in sufficient amount to cause product organic acid and weak contaminant to be displaced over a period of time from the complexed third immiscible basic extractant to produce a weak contaminant effluent stream that comprises a major amount of weak contaminant and a product organic acid stream that comprises a major amount of the product organic acid. Preferably the purified product organic acid fraction comprises between about 95% by weight and 99.5% by weight of the product organic acid that was present in the feed stream.
Certain embodiments are directed to a process for purifying an organic acid that comprises providing an aqueous feed stream comprising a product organic acid and one strong contaminant. The molar concentration of the product organic acid is at least 20 times greater than the molar concentration of the strong contaminant. The aqueous feed stream is contacted with a first immiscible basic extractant that has a selectivity for the strong contaminant relative to the product organic acid that is greater than about 3. The majority of the strong contaminant and less than about 33 wt % of the product organic acid become complexed with the first immiscible basic extractant. The complexed first immiscible basic extractant is separated from the aqueous stream, thereby producing a first effluent stream that comprises product organic acid and that has a greater ratio of product organic acid to strong contaminant than the aqueous feed stream did. The complexed first immiscible basic extractant is exposed to at least one of (1) a change in temperature, (2) a change in solvent concentration, or (3) a change in displacing agent concentration. The exposure causes product organic acid and strong contaminant to be displaced over a period of time from the complexed first immiscible basic extractant, thereby producing a second effluent stream that comprises a major amount of product organic acid and a third effluent stream that comprises a major amount of strong contaminant. The change in solvent concentration can be achieved using methods known in the art, such as evaporation solvents present in the aqueous feed stream. Furthermore combinations of temperature change, solvent change, and/or displacing acid or base concentration can be used to obtain selective release of desired product or of the impurities from the complexed first immiscible basic extraction. The displacing agent can be either a displacing acid as discussed above, or a displacing base. A base, such as NaOH, can be used as a displacing agent, and the displaced material can then be treated using methods known in the art to recovered the desired product organic acid.